Clostridial neurotoxins are the most potent protein toxins known. The neurotoxin produced from Clostridium tetani (tetanus toxin) is encountered by humans as a result of open wounds. However, tetanus poisoning at least in industrial countries is no longer a major public health problem due to the availability and widespread use of a safe, effective and inexpensive vaccine. This vaccine is basically a formalin-inactivated culture supernatant from C. tetani grown in fermentors.
Botulinum neurotoxin (BoNT), which is produced by the organisms Clostridium botulinum, Clostridium butyricum and Clostridium baratii, is the potent etiologic agent associated with the disease botulism (Simpson, L. Annu. Rev. Pharmacol. Toxicol. 1986 26:427-453). Humans are usually exposed to this neurotoxin through food poisoning, although there are rare incidents of wound botulism. A similar vaccine to the tetanus vaccine has been developed to provide protection from botulinum toxin poisoning. However, since there are seven different serotypes of botulinum toxin, complete protection with this inactivated toxin can be afforded only by making seven distinct vaccines and combining them for administration. Presently, only five of the seven serotypes are represented in the botulinum toxin vaccine. Further, some of the serotypes are composed of strains that do not produce high levels of toxin in culture. Thus, growth, purification and inactivation of the toxins for vaccine purposes is time consuming and expensive, owing to the high hazards associated with handling fully active toxin (Clayton et al. Infection and Immunity 1995 63(7):2738-2742). At this time this vaccine is only available through the Center of Disease Control for primarily experimental use.
Typically, botulism results from ingestion of food that is tainted with the toxin, or by the ingestion of food contaminated with organisms that can manufacture the toxin in the gut. Regardless of origin, botulinum toxin is synthesized as a relatively nontoxic single chain polypeptide with a molecular weight of approximately 150 kDa. To become fully toxic, it must undergo posttranslational processing, during which the molecule is cleaved by a protease to yield a dichain structure in which a heavy chain (approximately 100,000 daltons) is linked by a disulfide bond to a light chain (approximately 50,000 daltons). The dichain molecule is the holotoxin that accounts for biological activity. BoNT translocates from the gut into the general circulation (lymph and blood) wherein it is then distributed to cholinergic nerve endings which are the target sites of toxin action. The toxin enters these nerves, where it acts as a zinc-dependent endoprotease to cleave polypeptides that are essential for exocytosis (Montecucco, C. and Schiavo, G. Mol. Microbiol. 1994 13:1-8). Cleavage of these polypeptides leads to blockade of transmitter release and paralysis.
The heavy chain of the toxin is believed to be essential for binding and translocation of the toxin from the outside to the inside of the cholinergic nerve endings, while the light chain possesses the zinc-dependent endoprotease activity that accounts for the ability of the toxin to poison cholinergic nerve endings (Neimann et al. Behring Inst. Mitt. 1991 89:153-162). Accordingly, vaccines against botulism comprising a nontoxic 50 kDa carboxyterminal fragment of Clostridium botulinum have been described. LaPenotiere et al. Toxicon 1995 33(10):1383-6 and Clayton et al. Infection and Immunity 1995 63(7):2738-2742. Further, it has been suggested that this highly selective neurotoxin and tetanus toxin may be converted into nontoxic therapeutic tools that can be applied in delivery of drugs, hormones, enzymes or antiviral substances to the central nervous system.